introgression
Concise Oxford English Dictionary © 2008 Oxford University Press:
introgression /ˌɪntrə(ʊ)ˈgrɛʃ(ə)n/
▶noun Biology the transfer of genetic information from one species to another through hybridization and repeated backcrossing.
– origin C17: from L. intro- ‘to the inside’ on the pattern of egression, ingression.
'introgression' also found in these Oxford entries:

